This invention relates to a novel cathode-ray tube (CRT) having a field-emission cathode.
A CRT generally comprises an evacuated envelope having therein a target and means for producing one or more modulated electron beams in the envelope. The beam, or beams, is focused on and scanned over a target to perform a desired function. A beam-producing means, which is usually part of an electron gun, includes at least one cathode, which is the source of the electrons that are formed into a beam.
One type of cathode, referred to as a thermionic cathode, must be heated to a high operating temperature. A thermionic cathode requires a period of time to heat up after the tube is turned on and also dissipates power in order to maintain the high operating temperatures. The time delay for heating up and the power dissipation during operation are both undesirable features of a thermionic cathode. A beam-producing means comprising a thermionic cathode includes electrodes which may be separated from the cathode and from each other by fractions of a millimeter. These separations are fixed at room temperature, but must be maintained when the tube is operating. To achieve this, the structure must be designed to compensate for the heating effects resulting from the operation of the cathode at high temperatures.
Another type of cathode, referred to as a field-emission cathode, operates at about room temperature so that problems arising from high operating temperatures are completely avoided. Such a cathode employed in the beam-producing means of a CRT has been proposed. In one form, the cathode comprises a single point, or filament, from which electrons are emitted in response to an electric field produced by an associated electric-field-producing means. The current density that can be focused on the screen from such a source is inadequate for most CRT applications.
U.S. Pat. No. 3,866,077 to F. S. Baker et al proposes using an array of at least 1000 electron-emitting filaments in parallel in order to provide a composite beam with sufficient electron-beam current for most common uses of a CRT. While larger currents may be realized with this structure, the combined emissions of multiple filaments is too divergent to permit adequate focusing of the beam on the target of the CRT.
U.S. Pat. No. 3,921,022 to J. D. Levine proposes using a single protuberance having multiple points on the surface thereof which emit electrons in response to an electric field produced by an associated electric-field-producing means. Also included are means for producing a focusing field for the emitted electrons. Analysis indicates that the combined emission from this structure also is too divergent to permit adequate focusing of the composite beam on the target.